Traveling-wave CW ring dye lasers are capable of several watt single-frequency outputs because they may be pumped with the full power of available ion lasers. In contrast, an input power limit exists in a standing-wave or linear cavity dye laser due to the regions of unsaturated gain in the pumped volume of the dye jet at the nodes of the standing wave. It has been shown that the fraction of the total volume the unused portion represents, decreases as the dye beam intensity increases. The drop in volume, however, is less rapid than the linear rise in pump power. Thus a mode at a second frequency, which has antinodes where the first mode has nodes, must eventually reach threshold and oscillate as the pump level is increased in the standing wave case.
This limit does not exist in a ring laser, and typically a ring is pumped four times harder than a standing wave laser. Thus, a typical output power specification for a single frequency standing-wave laser is 150 mW output, using a 3 W argon ion laser as an optical pump operating at a wavelength of 514 nm, and using Rhodamine 6G dye as the lasing medium. A ring dye laser, however, can provide 1 W single-frequency output with a 6 W input pump beam. This is a major advantage of the ring configuration.
A second major advantage of a ring configuration is that the internal second-harmonic generation powers that can be attained are an order-of-magnitude greater than possible in a standing-wave configuration.
A ring laser cavity typically employes a four mirror, figure-eight configuration to keep the fold angles small, allowing astigmatic compensation with a Brewster plate of reasonable thickness. An important optical element in a ring laser is the "optical diode," a device which forces the laser to operate stably in the preferred, or forward traveling-wave direction. Obviously, the output beam is unusable if it randomly switches direction from horizontal, to several degrees below horizontal, which it does without an optical diode as the beam direction around the ring switches from forward to backward.
A typical optical diode consists of a Faraday cell surrounded by samarium-cobalt permanent magnets and a section of quartz-crystal cut for optical activity. (See S. M. Jarrett and J. F. Young, Paper D3, Digest of Technical Papers, Tenth IQEC, Atlanta, Ga (May 29-June 1, 1978), p. 634). The rotation of polarization of the dye beam through a small angle in the transit of the Faraday cell is "undone" for the forward wave in traversing the crystal. The backward wave undergoes a polarization rotation of approximately double the Faraday angle in transit of this pair of elements, and suffers a subsequent reflection loss at the Brewster surfaces in the cavity.
Both the Faraday rotation angle for a fixed magnetic field, and the rotation angle from the optically active plate, vary roughly as the inverse-square of the wavelength. The problem in designing a wide-band optical diode is one of keeping a close enough match of the Faraday and back-rotation angles to have an acceptably low insertion loss for the forward wave on the blue end of the useful spectrum, and at the same time to have enough Faraday rotation remaining on the red end to give the minimum required differential loss between forward and backward waves.
Existing optical diodes for ring lasers are limited in their spectral range. Further existing optical diodes typically use anti-reflection coated, normal incidence surfaces which limits operation, in any case, to the relatively narrow low-reflectivity band pass of the coating.